(a) Field of the Invention
The present invention relates to a four wheel drive vehicle with an anti-lock braking system comprising front and rear axles coupled together by a power transmission assembly capable of directly coupling the front and rear axles, a brake attached to each of the front and rear wheels, and a hydraulic braking system for controlling the hydraulic pressure of each brake applied to the wheels, the hydraulic braking system including an anti-lock control device arranged to control the hydraulic braking pressure so that the braking pressure may be reduced to prevent locking of the wheels.
The invention further relates to the method of operation of the anti-lock braking system.
(b) Description of the Prior Art
There has been a continuing development process with respect to improving various aspects of the characteristics and running performance of four wheel drive vehicles while traveling on road surfaces which have a low coefficient of friction, and in addition, attempts have been made to equip such four wheel drive vehicles with anti-lock braking system.
In general, the anti-lock braking systems of the prior art have no problems in cases where a four wheel drive vehicle is set in the following modes: a lock mechanism in non-operational mode in which the front and rear axles are coupled to a power unit by means of a differential gear including the lock mechanism; and a clutch disengagement mode in which either the front or the rear axle is coupled to the power unit and at the same time the other axle is coupled to the power unit by the clutch which is manually brought into engagement and disengagement.
However, the following problem occurs when a four-wheel drive vehicle with a prior art anti-lock braking system is set in a mode in which the differential gear is locked or the clutch is engaged. Specifically, during a four wheel drive operation when the braking force applied to the front wheels is transmitted to the rear wheels, there is a tendency for mutual interference between the front wheels and the rear wheels, thereby making it difficult to achieve a satisfactory anti-lock effect.